1. Field of the Invention
The invention relates to an apparatus for manufacturing single crystal by use of a weighing control in diameter method.
2. Description of Prior Art
The Czochralski (hereafter, CZ) method is one of the methods for manufacturing single crystal in the semiconductor industry. In the CZ method, polysilicon with high purity fed into a quartz crucible is melted into a melt, a seed is dipped into the melt and then the seed stuck in the melt is pulled to form single crystal silicon. FIG. 4 depicts the basic structure of an apparatus for manufacturing single crystals based on the CZ method. A seed 22 supported by a seed holder 21 which is connected to a pulling wire 19 is rolled up by a wire-rolling drum 5, thereby pulling the growing single crystal silicon. In the apparatus for manufacturing single crystal by using the pulling wire 19, the predetermined dimension (diameter) of the single crystal silicon can be controlled by an optical control in diameter method. In the optical control in diameter method, a meniscus ring on the interface 24 of the melt and single crystal silicon (not shown) is recorded by a camera 23, wherein the generated video signals are fed into a width-calculating unit 26, thereby calculating the diameter of the single crystal silicon located at the meniscus ring, and a monitor 28 from a camera control unit 25. Then the diameter of the single crystal silicon can be controlled at a predetermined value based on calculated pulling speed signals 29 and estimated melting temperature signals 30 for controlling the diameter of the single crystal silicon by the diameter control device 27.
As another control method based on the CZ, a weighing control in diameter method, is well known. In the weighing control in diameter method, the weight of the growing single crystal silicon measured by a load cell is compared with a standard crystal weight set by the diameter control device. Based on the difference between the weight measured by the weighing control in diameter method and the standard crystal weight, the pulling speed and melting temperature are adjusted, thereby keeping the diameter of the single crystal silicon approximately equal to the diameter of the standard crystal. In this method, since the weight of the growing single crystal silicon must be accurately measured, a special apparatus for manufacturing single crystal silicon is utilized. This apparatus can directly measure the weight of the single crystal silicon by suspending the seed at a loading point of a load cell via a force bar
The weighing control in diameter method has superior diameter control characteristics to those of the optical control in diameter method. Particularly, it has excellent reproduction of absolute value in diameter and controllability of a formed tail (i.e., the reduction in the diameter of the tail portion of the single crystal silicon during growing can be performed). However, since a force bar is used in the weighing control in diameter method while a pulling wire is employed in the optical control in diameter method, when manufacturing the same length of the single crystal silicon by the weighing control in diameter method, the manufacturing apparatus must be twice as high as that required for the optical control in diameter method. As a result, the weighing control in diameter apparatus occupies more space than that of the optical control in diameter apparatus. Furthermore, the cost of the apparatus for the former method is greater than that for the latter. To resolve the above-mentioned problem, it is necessary to apply the weighing control in diameter method to a typical apparatus by using a pulling wire to pull the single crystal silicon.
As briefly shown in FIG. 5, in an apparatus for manufacturing single crystal silicon 33 by using a pulling wire, the structure for measuring the weight of the single crystal silicon comprises a guide pulley 32 disposed above a wire-rolling drum 31, wherein the weight of the single crystal silicon is applied on the guide pulley 32 via a pulling wire 35 and a load cell 34 for measuring the weight of the single crystal silicon. However, the precise weight of the single crystal silicon required by the weighing control in diameter method can not be measured by this structure. The reason why it is not possible to measure the weight of the single crystal silicon precisely will be described in the following. Since the flexible pulling wire 35 is formed by twisting filaments, the diameter of the pulling wire 35 is not a constant. Therefore, when using this structure shown in FIG. 5 to measure the weight of the single crystal silicon, the distance between the center point on the guide pulley 32 and the cross-sectional center point of the pulling wire 35 (i.e., the point on which the weight of the single crystal silicon is applied) is defendant on the diameter direction of the guide pulley 32. Due to this variability, the load center of the guide pulley 32 also varies. Consequently, the balance point of a force applied to the guide pulley 32 departs from the load applied on the load cell 34, making it impossible to weigh the single crystal silicon accurately. Due to this phenomena caused by the variations in diameter of the pulley wire 35, the measured weight of the load cell 34 varies periodically, depending on the twisting pitch of the pulling wire 35 and the rolling speed of the wire-pulling drum 31, even though there is in fact no variation in the weight of the single crystal silicon. With such an error in the weight of the single crystal silicon measured by the load cell, the diameter of the single crystal silicon can not be controlled stably and the high quality single crystal silicon can not be obtained.